This application is based on Japanese Patent Applications 2000-278638, filed on Sep. 13, 2000, and 2000-21684 filed on Jan. 31, 2000, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to a semiconductor device and a semiconductor laser, and more particularly to a semiconductor device in which a strained layer is formed on a substrate having a semiconductor surface, the stained layer being made of semiconductor material having a lattice constant different from that of the semiconductor surface.
b) Description of the Related Art
Developments on semiconductor crystal growth methods typically organo-metal or metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) have made it possible to grow a semiconductor layer on a substrate, the semiconductor layer having a lattice constant different from that of the substrate and containing strain.
A strain amount or film thickness of a strained layer to be used in a semiconductor device has a critical value. This critical value is called a critical strain amount or critical film thickness. Misfit (crystal) dislocations occur if a strain stress exceeds an elastic limit specific to each kind of crystal, and lower the performance of the semiconductor device. Even if there is no misfit dislocation immediately after the manufacture of semiconductor devices, misfit dislocations may occur by a thermal history or the like after the long term use. A sufficient margin of the critical strain amount or film thickness is required in order to improve the reliability of semiconductor devices.
These limits in the strain amount or film thickness of a strained layer restrict the design of semiconductor devices such as strained quantum well semiconductor lasers and high electron mobility transistors (HEMT). Techniques of suppressing the generation of vacancies in a strained layer by using a graded substrate are disclosed, for example, in JP-A-7-312461. The effects of these techniques, however, are considered unsatisfactory.
From the restrictions of lattice constant, an InP substrate is generally used for a semiconductor laser using an InGaAsP strained quantum well and oscillating at a wavelength of 1.3 xcexcm. If a GaAs substrate is used, although the good temperature characteristics at a higher temperature can be expected, the necessary strain amount in the InGaAsP strained layer exceeds the critical value. It has been therefore difficult to use a GaAs substrate for an InGaAsP semiconductor laser.
A blue semiconductor laser uses In0.12Ga0.88N and In0.03Ga0.97N as the materials of a well layer and a barrier layer in a multiple quantum well (MQW) structure and GaN as the material of a light confinement layer. A substrate lattice-matching with these materials is not known and it is inevitable to use a strained layer. Designs are therefore restricted by the critical strain.
It is an object of the present invention to provide a semiconductor device having a structure allowing to form a high quality strained layer in terms of crystallography.
According to one aspect of the present invention, there is provided a semiconductor device comprising: a substrate having a principal surface exposing a first semiconductor material; a micro structure disposed on the principal surface of the substrate, the micro structure being made of a second semiconductor material having a lattice constant different from a lattice constant of the first semiconductor material, and defining a three-dimensionally irregular upper surface; and a strained layer disposed on the micro structure, the strained layer being made of a third semiconductor material having a lattice constant different from the lattice constant of the first semiconductor material.
Since the micro structure is disposed between the strained layer and substrate, the strained layer can be grown to be thicker than a critical film thickness. The reason for this may be ascribed to that the micro structure forms a variation in the directions of vectors of a strain stress in the strained layer.
According to another aspect of the present invention, there is provided a semiconductor device comprising: an underlying substrate made of semiconductor material, the underlying substrate having irregular in-plane lattice constants on an upper surface of the underlying substrate; and a strained layer formed on the upper surface of the underlying substrate, the strained layer being made of semiconductor material and containing strain.
Since the in-plane lattice constants on the upper surface of the underlying substrate are irregular, there is a variation in the directions of vectors of a strain stress in the strained layer.
According to another aspect of the present invention, there is provided a semiconductor laser comprising: a substrate made of semiconductor material of a first conductivity type; a micro structure disposed on the substrate, the micro structure having irregular in-plane lattice constants on an upper surface of the micro structure; a strained active layer disposed on the micro structure, the strained active layer being made of semiconductor material and containing strain; a clad layer formed on the strained active layer, the clad layer being made of semiconductor material of a second conductivity type opposite to the first conductivity type, the semiconductor material of the second conductivity type having a lattice constant equivalent to a lattice constant of the substrate; and electrodes for allowing a current to flow between said substrate and said clad layer.
Since the micro structure is disposed between the substrate and the strained layer, the strained active layer can be grown to be thicker than the critical film thickness. As the strained active layer is made thick, the emission wavelength becomes long. Therefore, the degree of freedom of selecting an emission wavelength can be increased.
According to another aspect of the present invention, there is provided a semiconductor device comprising: a substrate having a principal surface exposing a first semiconductor material; an underlying layer disposed on the principal surface of the substrate, the underlying layer being made of a second semiconductor material containing In, the second semiconductor material having a lattice constant which lattice-matches a lattice constant of the first semiconductor material in a range of 1% or lower; and a strained semiconductor layer disposed on the underlying layer, the strained semiconductor layer being made of a third semiconductor material having a lattice constant different from a lattice constant of underlying semiconductor material.
In is segregated on the surface of the layer containing In. A semiconductor film thicker than the critical film thickness can be grown on the In containing layer without generating misfit dislocations.
For example, a micro structure or a semiconductor layer including In is disposed between an underlying substrate and a strained layer. When the micro structure or the semiconductor layer including In is disposed, the strained layer can be grown thicker than a strained layer directly grown on the underlying substrate. Therefore, design freedom of the semiconductor device using a strained layer can be improved. More reliable semiconductor device can be obtained.